The field of the invention generally relates to recuperative or condensing furnaces, and more particularly relates to apparatus and method for elevating the dew point of combustion products before entry into a condensing heat exchanger.
As is well known, nonrecuperative furnaces transfer only sensible heat from the combustion products. That is, condensation does not occur within the primary heat exchanger because the combustion products are exhausted at a temperature above their dew point. Accordingly, heat transfer by nonrecuperative or noncondensing furnaces is commonly referred to as a dry process.
In contrast, recuperative furnaces not only transfer sensible heat, but also cool the combustion products below their dew point so that heat of condensation is also transferred to the exchange medium. The additional transfer of heat by a recuperative heat exchanger has the advantage of increasing the overall furnace efficiency such as, for example, to approximately 95% whereas nonrecuperative furnaces are generally limited to less than 90%. Besides providing high efficiencies, the lower exhaust temperatures of recuperative furnaces enable the use of inexpensive exhaust venting such as, for example, PVC pipe rather than conventional chimneys.
Recuperative furnaces, however, are subject to corrosive attack of the recuperative heat exchanger by acidic condensate forming therein. In combusting natural gas, and to a greater extent fuel oil, a number of potentially acidic forming gases are produced. Although these gases are typically noncondensable at the operating temperatures of a recuperative heat exchanger, they are absorbed by water vapor condensate thereby forming acids. For example, carbon dioxide forms carbonic acid, nitrogen dioxide forms nitric acid, hydrogen chloride forms hydrochloric acid, and hydrogen fluoride forms hydrofluoric acid. In addition, sulfur dioxide will condense within a recuperative heat exchanger thereby forming sulfurous acid. The acidity of the condensate is further increased when water condensate evaporates leaving behind concentrated acids which corrosively attack the heat exchanger.
Corrosive attack may also occur on heat exchange surface areas which are only exposed to combustion products that are above their dew point temperature. At the beginning of the heating cycle, incipient condensation may briefly form on initially cool surface areas. As these surfaces become heated during the heating cycle, the condensation evaporates and does not reoccur. Localized corrosion may therefore occur on these surfaces.
There have been a number of prior art attempts to prevent heat exchanger damage caused by corrosive attack. In one approach, stainless steel components have been used because they are less susceptible to corrosion. Such heat exchangers, however, are very expensive. In order to limit the cost, heat has been transferred from the combustion products in stages wherein only the final stage heat exchanger is recuperative and therefore stainless steel only needs to be used for a relatively small condensing heat exchanger during a final stage. However, such arrangement introduces the complexity of having multiple combustion product heat exchangers. Further, it has been found that chlorides are often present in the environment at levels which produce sufficient hydrochloric acid to corrode even stainless steel. A stainless steel molybdenum alloy may be resistant to hydrochloric acid, but such material is prohibitively expensive for residential heat exchangers. In another approach, the condensing heat exchanger is flushed with pure water to rinse away acids after each firing of the burner. Such arrangement, however, puts constraints on the type of heat exchanger that can be used, and also increases the complexity and cost of the system.
My U.S. Pat. No. 4,681,085 describes a recuperative or condensing furnace wherein the dew point of combustion products is elevated above their natural dew point before introducing them into a combustion product heat exchanger. Accordingly, the formation of condensate in the recuperative heat exchanger is greatly increased, and the condensate runs downwardly in counterflow to the combustion products thereby continuously flushing away and preventing high concentrations of acid. Because a significant amount of condensate flows downwardly, the inner surfaces of the combustion product flow path through the heat exchanger are kept continuously wet. Thus, transition regions between wet and dry surface areas are eliminated or greatly reduced; these transition regions were found to exhibit high corrosion. Further, there was less corrosive attack because the temperature of the combustion products was lowered in the process of elevating the dew point before entering the heat exchanger. Thus, the surface areas of the heat exchanger were not heated to so high a temperature.
The dew point was described as being raised by providing a liquid containing reservoir adjacent the input of the heat exchanger and using a radiant burner wherein approximately 50% of the generated heat is radiant heat which is directed toward the liquid to raise its temperature. Such technique, however, is rather expensive because radiant burners are relatively costly to fabricate, and other methods require additional apparatus. That is, alternatively, the dew point was described as being raised by using a water atomizer or by spraying particles of water into the flow of combustion products.